Mapping requests to localhost servers based on Accept header

nginx.conf

events {
    worker_connections  1024;
}

http {
    map $http_accept $accept {
        default app;
        ~text/html web;
    }

    upstream app {
        server localhost:8888;
    }

    upstream web {
        server localhost:3000;
    }

    server {
        listen 8080;
        server_name localhost;
        location / {
            proxy_set_header Host $http_host;
            proxy_set_header X-Forwarded-For $remote_addr;
            proxy_pass http://$accept;
        }
    }

}